The application of special, heat-sensitive inks to an object, on the surface of which a drawing or inscription is embodied, and subjecting these to heat or radiation is known to produce thickening of the drawing or inscription outline, which remains raised with respect to the rest of the surface.
These types of ink can be used for merely decorative purposes to confer a distinctive character on garments, cartons, metal plates or papers, amongst others, or can also be used to transmit a message written in Braille for recognition by, and the education of, blind people.
Invention patent EP 0 526 396 indicates that the ink may be colourless or of a specific colour or colours, is applied to a surface and subjected to drying at a lower temperature than that of expansion, after which it is possible to apply a varnish, and is finally subjected to energetic radiation, such as laser radiation, to heat the surface, which causes expansion of the ink.
Similarly, printing systems are applied to notes of legal tender on which ink layers of adequate thickness form figures that remain embossed to facilitate their identification by blind people. Said embossing stands out in prominent relief with respect to the horizontal surface of the note, due to which it suffers wear as a consequence of handling, finally making the embossing barely perceptible to blind people, who will be unable to recognize said note.
On the other hand, the use of expandable microspheres in paper production is known to increase its volume and thickness, while maintaining original weight and rigidity. Volume increase in the range of 18% to 22% can be obtained by using said microspheres, thereby achieving substantial savings on the cost of the pulp required to obtain the same volume.
These types of microsphere have a diameter of approximately 10 to 12 microns and a thermoplastic coating composition that encompasses an expanding agent in such a way that, when said microspheres are heated, the thermoplastic coating becomes elastic, the internal agent becomes gasified and the microsphere expands up to 4 or 5 times its original diameter.
We must point out that the microspheres are mixed with the paper fibres and tend to expand at a given temperature, before the union between said fibres is sufficiently rigid so as to prevent said expansion, the microspheres being sufficiently resistant so as to support the mechanical tensions that occur at higher temperatures.